Since 9-11, carrying a pocketknife can be tricky business. However, with rare exception, nail trimmers are allowed most places that check for or restrict weapons. I have found that nail trimmers can be a handy tool for so many things beside personal grooming.
Clipping a stray thread, cutting strings, and tape, or even (maybe) cutting out a coupon. OK, we all know we'd just tear that one, but work with me here... these really can be a useful thing to have on hand.
And nothing bothers me more than chipping a nail with no way to fix it.